= image of the physical aperture of the lens , as mapped by the lens elements between aperture and object. For front-aperture-lenses, entry pupil and front aperture are identical.

This is the apparent opening that we see as humans when we look towards lens from the object side.

What we see here is _not_ the mechanical iris, but just an image of the iris as seen through the lenses in front of the mechanical iris. You can imagine the lenses in front of the iris as kind of a magnification glass, through which we see the mechanical iris.

Diameter and position of the entry pupil influence the F-number and thus the light sensitivity of a lens and also the angle of view.

The diameter of the entry pupil alone does _not_ describe the F-number (=measure for the ability to collect light)! Only comparison lenses of the _same_ focal length allow to directly decide which Lens has the lower F-number

Light of _each_ object point in the field of view of the lens reaches _each_ point on the front lens of the objective, but only the light directed to the entry pupil of the lens reaches the matching image point.

Rays from the object point directed to the center of the entry pupil are called “chief ray”.

The (object side) chief ray angle is the angle between optical axis of the lens and all chief rays. The viewing angle of the lens is twice max chief ray angle.

Here a 3D Graphics that shows in yellow the light from the Objetpoint that heads the Entry pupil and on its way hit’s the front lens of the system. Move the green point.

The example above is over emphasized. Here a more realistic example with a front iris, which also acts as a entry pupil: